1. Field of the Invention
This invention relates to three-color plasma-panel display devices, fluorescent lamps and display devices equipped with low-energy electron sources.
2. Description of the Prior Art
The 253.7 nm radiation from mercury vapor has hitherto been widely and generally utilized for exciting phosphor screens. In recent years, however, it has been recognized that mercury is detrimental to the human body, and the development of light emitting devices employing no mercury has been being carried forward. In, for example, a fluorescent discharge lamp for illumination, it is desirable to excite a phosphor by ultraviolet radiation shorter than 253.7nm, but being preferably chosen to be between 58.4nm and 250nm. The first resonance line and ionization potential of helium gas lie at 58.4nm and 50.4nm. In terms of the energy of photons, such wavelengths correspond to an energy range of 5eV to 25 eV.
In a display tube equipped with a low-energy electron source, the upper limit of the accelerating voltage should desirably be 25 V in consideration of the first ionization potential 24.581 eV (50.4nm) of helium and also the maximum plasmon energy of various phosphors to be utilized for a solid luminescent screen.
A gas discharge cell which contains phosphors therein and in which they absorb the energy of gas plasma to emit different colors of light, has been known from U.S. Pat. No. 3,559,190. An attempt to obtain multicolor in plasma-display devices containing well-known phosphors and vacuum ultraviolet radiation sources has been reported in "Three-Color-Plasma-Panel-Display Device" prepared by F. H. Brown, C. W. Salisbury, H. G. Slottow and M. J. Tamm, Owens-Illinois, Inc., Ohio, under Contract No. DAAB 07-70-C-0243, December, 1970). Therefore, it will be regarded as being already known to use, for example, lead activated calcium tungstate phosphor (CaWO.sub.4 :Pb) for blue of the three primary colors, manganese activated zinc silicate phosphor (Zn.sub.2 SiO.sub.4 :Mn) for green, and europium activated yttrium vanadate phosphor (YVO.sub.4 :Eu) for red.
Where the above-mentioned lead activated calcium tungstate is employed for a blue emitting device associated with this invention, the following two disadvantages are involved. The first is that although the phosphor has comparatively high luminous efficiency under 253.7nm excitation and also usual X-ray excitation, the luminescence when excited by 5 to 25 eV photons or electrons is feeble. For example, it is difficult to obtain an intensity ratio of more than 70% under 147 nm excitation relative to the luminescence intensity under 253.7 nm excitation even by controlling the shape of the individual phosphor particles and also by optimizing the activator concentration. The second is that the emission spectrum of the phosphor is very wide and then the purity of blue color is extremely inferior as will be stated later.
On the other hand, where the europium activated yttrium vanadate phosphor described in the report is employed for a red light emitting device associated with this invention, the following two disadvantages are involved. The first is that although the phosphor has comparatively high luminous efficiency under 365nm (3.4 eV) or 253.7nm (4.8 eV) excitation and also under the usual electron beam excitation of 8 - 25 eV, the luminescence when excited by 5 to 25 eV photons or electrons is feeble. For example, it is difficult to obtain an intensity ratio of more than 60% under 147nm excitation relative to the luminescence intensity under 253.7nm excitation even by controlling the shape of individual phosphor particles and also by optimizing the activator concentration. The second is that where the phosphor is used in combination with a well-known phosphor, such as manganese activated zinc silicate phosphor and any suitable blue phosphor in a three-primary-color plasma display device which is operated by an identical discharge current, it is difficult to establish the white balance because the green phosphor is very good in color purity and reveals high brightness.
When the excitation energy of 5 - 25 eV according to the present invention is compared with that of the photons or electrons widely employed, there exists two peculiar points stated hereunder. As the first peculiarity, the excitation energy is higher than that of characteristic absorption bands (3 - 5 eV) of various activators or sensitizers to be added to the phosphors, and belongs to an energy region equal to or higher than the energy of the fundamental absorption edge of various phosphors. The second peculiarity is that the excitation energy belongs to an energy region equal to or lower than the plasmon energy in a number of insulating inorganic phosphors. Therefore, when a well-known blue phosphor such as zinc sulfide activated by silver and co-activated by chlorine (ZnS:Ag:Cl) is used jointly with the exciting source of this invention, the phosphor does not reveal high efficiency, because it is difficult to convert transmission of the plasmon energy effectively to the activator. The situation is the same with a red phosphor, for example, europium activated yttrium oxide (Y.sub.2 O.sub.3 :Eu), yttrium oxysulfide (Y.sub.2 O.sub.3 S:Eu) and yttrium vanadate (YVO.sub.4 :Eu). In order to get efficiency under excitation of 5 - 25 eV, a phosphor composition having a unique property in the absorption and relaxation processes of the excitation energy is accordingly requested.